1. Field of the Invention
This invention relates to a heat transfer tube used for an absorber, a regenerator or an evaporator of an absorption refrigerating machine, and more particularly to a heat transfer tube having grooves or irregularities on the circumferential surface for use in an absorption refrigerating machine.
2. Description of the Prior Art
As shown in FIG. 19, an absorption refrigerating machine in general has an evaporator 4, an absorber 5, a regenerator 6 and a condenser 7.
In the evaporator 4 approximately under vacuum, heat transfer tubes 40 are arranged in a horizontal state at predetermined intervals in the vertical and horizontal directions, and the vertically adjacent heat transfer tubes 40 are communicated with each other.
A refrigerant (water) 44 supplied from the condenser 7 or a refrigerant pipe 41 having a refrigerant pump 42 is spread over the outside surface of the heat transfer tube 40 for the evaporator through a spreader pipe 43. Water flowing through the inside of the heat transfer tube 40 is cooled down by the refrigerant 44 flowing downwards along the surface of the heat transfer tube 40.
In the absorber 5 and the regenerator 6, heat transfer tubes 50, 60 are respectively arranged in a horizontal state at predetermined intervals in the vertical and horizontal directions, and the vertically adjacent heat transfer tubes 50, 60 are respectively communicated with each other.
An absorbent (aqueous solution of lithium bromide) is spread over the outside surface of the heat transfer tube 50 for the absorber through a spreader pipe 51. A refrigerant (water) flows through the inside of the heat transfer tube 50 and is supplied to a heat transfer tube 70 arranged in the condenser 7.
The refrigerant 44 is evaporated due to the temperature of water flowing through the inside of the heat transfer tube 40, and the resultant vapor of the refrigerant 44 is absorbed into a low-temperature absorbent 52 flowing downwards along the surface of the heat transfer tube 50 in the absorber 5. The absorbent 52 having the reduced concentration resulting from the absorption of the refrigerant vapor is sent to a spreader pipe 61 in the regenerator 6 using a pump 53.
The low-concentration absorbent 52 sent to the spreader pipe 61 is spread over the surface of the heat transfer tube 60 for the regenerator through the spreader pipe 61. While the absorbent 52 flows downwards along the surface of the heat transfer tube 60, the refrigerant absorbed into the absorbent 52 is boiled up by a heating medium flowing through the inside of the heat transfer tube 60, and as a result, separated from the absorbent 52.
The refrigerant vapor separated from the absorbent 52 by the regenerator 6 is cooled down for condensation through the heat transfer tube 70 in the condenser 7. The condensed refrigerant 44 is returned to the evaporator 4, and then spread over the heat transfer tube 40 through the spreader pipe 43.
On the other hand, the absorbent 52 regenerated by the regenerator 6 is cooled down by a heat exchanger 54, and subsequently returned to the absorber 5.
According to the circulation described above, water flowing through the inside of the heat transfer tube 40 of the evaporator 4 can be continuously cooled down.
Recently, with the demand of a smaller-sized and higher-performance absorption refrigerating machine, a smaller-diameter and higher-performance heat transfer tube has been required for the absorption refrigerating machine.
The heat transfer tubes used for the evaporator 4, the absorber 5 and the regenerator 6 are adapted for the transfer of heat between a fluid inside the heat transfer tube and a medium (the absorbent 52 or the refrigerant 44) flowing downwards along the surface of the heat transfer tube while keeping in contact with the same. Thus, in order to provide a smaller-sized heat transfer tube and to improve the heat transfer performance thereof, it is necessary to wet the surface of the heat transfer tube with the medium throughout as much as possible. Namely, it is necessary to accelerate the diffusion of the medium over the surface of the heat transfer tube and the expansion of the surface area of the heat transfer tube wet with the medium (or the improvement in wettability).
In addition, heat is transferred on the contact surface between the heat transfer tube and the medium in most cases. Thus, when the medium flows downwards along the surface of the heat transfer tube, it is necessary to further activate the convection of the medium (interfacial turbulence or disturbance of liquid membrane).
As for a heat transfer tube having a structure to accelerate the expansion of the surface area wet with a medium flowing along the circumferential surface and the disturbance of a liquid membrane, for example, Japanese Utility Model Laid-open No. 57-100161 (Invention by Masaki Minemoto) has disclosed a heat transfer tube for an absorber, in which a large number of small grooves are formed helically on the circumferential surface of the tube.
The heat transfer tube described in the above Publication is constituted to flow the absorbent along the helical grooves on the surface of the tube. Thus, the absorbent is substantially diffused in the axial direction (length direction) of the tube, and as a result, the wet area on the surface of the tube is expanded. In this manner, this heat transfer tube has been intended to improve the heat transfer performance and to provide a smaller-sized apparatus.
In addition, as for another heat transfer tube having a structure to accelerate the interfacial turbulence of a medium, for example, Japanese Patent Laid-open No. 63-6364 (Invention by Giichi Nagaoka and others) has disclosed a heat transfer tube for an absorber, in which a large number of projections each having a height of 2 mm are formed on the circumferential surface of a blank tube having an outer diameter of 19 mm in parallel to the tube axis, and each projection is notched at a depth of 0.5 mm at pitches of 5 mm.
The present inventors manufactured an experimental apparatus composed of a pair of supports capable of horizontally supporting five heat transfer tubes at intervals of 6 mm in the vertical direction, and a spreader pipe arranged to be spaced above by 25 mm from the uppermost heat transfer tube supported by the supports. In this case, a heat transfer tube manufactured on trial similarly to each of the prior art heat transfer tubes was used as each of five heat transfer tubes in the experimental apparatus. Then, the present inventors made observations of the flow state of red ink on the surface of the heat transfer tubes and the wet state of the heat transfer tubes, while continuously spreading the red ink through the spreader pipe.
As a result, in case of using the heat transfer tubes described in Japanese Utility Model Laid-open No. 57-100161, it was confirmed that the red ink flows in the axial direction (length direction) of the tube along the helical grooves due to the gravity in the range of each heat transfer tube from the top surface to the side surface, while the ink reaching to the side surface of the tube stops flowing along the helical grooves, and most ink drops across the ridges on both sides of each groove in the course of the process of flowing the ink downwards. Namely, a considerable surface area on the underside of the tube was not wet.
Further, the diffusion of the ink in the axial direction of the tube was inferior on the top surface of the tube as well.
On the other hand, in case of using the heat transfer tubes described in Japanese Patent Laid-open No. 63-6364, the ink was substantially diffused in the axial direction of the tube along the projections on the surface of the heat transfer tube. When the ink was collected between the mutually adjacent projections (grooves) up to the notches of the projections, the ink was moved from the notch portions of the projections to the next groove in the circumferential direction of the tube, and further diffused in the axial direction of the tube along the groove. Namely, the surface of the tube was satisfactorily wet as a whole.
However, in case of making the observations of the latter heat transfer tubes from a viewpoint of the interfacial turbulence, the liquid membrane was satisfactorily disturbed in the circumferential direction of the tube. On the other hand, since the shape of each groove between the mutually adjacent projections is uniform in the length direction, the liquid membrane was not satisfactorily disturbed in the axial direction of the tube.